1. Field of the Invention
This invention relates to the field of optical devices that manipulate optical energy of tightly controlled optical wavelength, particularly for use in communication applications. More particularly, the invention relates to lasers which produce optical energy of a specified wavelength and which can be tuned or switched to other specified wavelengths by thermal means.
2. Description of the Related Art
Over the past several years, there has been ever-increased interest in tunable lasers for use in optical communication systems, in general, and for use in dense wavelength division multiplexing (DWDM) applications, in particular. DWDM allows high bandwidth use of existing optical fibers, but requires components that have a broad tunable range and a high spectral selectivity. Such components include tunable lasers that should be able to access a large number of wavelengths within the S-band (1490-1525 nanometers), the C-band (1528-1563 nanometers), and the L-band (1570-1605 nanometers), each different wavelength separated from adjacent wavelengths by a frequency separation of 100 MHz, 50 MHz, or perhaps even 25 MHz.
The distributed Bragg reflector (DBR) laser was the first such tunable laser used in optical communication. The DBR laser consisted of a semiconductor amplifier medium, defining an active section, and an optical waveguide. The optical waveguide included a portion without a grating that defined a phase control section and a portion in which a single grating of typically constant pitch (Λ) was formed which constituted a distributed Bragg reflector or, more simply, the Bragg section that reflected light at the Bragg wavelength λB. Wavelength tuning of such a DBR laser was performed by transferring heat into the phase control section, the Bragg section, or both. The optical waveguide was defined by an organic layer which constituted a core with another organic confinement layer disposed both above and below the core. Wavelength tuning of such a DBR laser was performed by either injecting current or transferring heat into the phase control section, the Bragg section, or both. Injecting minority carriers made it possible to vary the refractive index of the waveguide and thus control the Bragg wavelength λB by the equation λB=2neff Λ where Λ is the pitch of the grating and neff is the effective refractive index of the waveguide. Alternatively, a pair of heating resistance strips was disposed on opposite outer surfaces of the laser component for the phase control section, the Bragg section, or both. By independently controlling the voltages to the heating resistance strips, the temperature and hence the index of refraction of the organic layers that form the optical waveguide was controlled via the thermo-optical effect. Tuning by injecting current had the disadvantage of increasing optical loss and adding optical noise. Tuning by heating had the disadvantage of increasing optical loss and adding optical noise. Both options induce long-term drift in the Bragg wavelength thereby reducing reliability. For a more detailed discussion of a wavelength tunable DBR laser by heating, please refer to U.S. Pat. No. 5,732,102 by Bouadma entitled “Laser Component Having A Bragg Reflector of Organic Material, And Method of Marking It” which is hereby incorporated by reference.
A super structure grating distributed Bragg reflector (SSG-DBR) laser was another type of tunable laser that held great promise. The InGaAsP-InP SSG-DBR laser was comprised of a semiconductor amplifier medium with an InGaAsP/InGaAsP multiple quantum wells active region, an SSG-DBR section on both sides of the semiconductor amplifier medium, and a phase control section between one of the SSG-DBR sections and the semiconductor amplifier medium. Thin film Pt heaters were formed on the top surface and corresponding electrodes were formed on the bottom surface of each SSG-DBR section and the phase control section. The two SSG-DBR sections were used as mirrors with different sampling periods giving different peak separations and different reflective combs in the reflectivity-wavelength spectrum. In the reflectivity-wavelength spectrum, only one reflective peak associated with each SSG-DBR section coincided and where these reflective peaks coincided at a cavity mode, that cavity mode was selected for lasing. Wavelength tuning of the SSG-DBR laser was performed by injection current into or heating of either SSG-DBR section or the phase control section. Current injection into or heating of the SSG-DBR sections changed the refractive index of each waveguide and shifted the reflection spectrum of each SSG-DBR section. Similarly, current injection into or heating the phase control section shifted the cavity modes. While providing a broad tuning range, wavelength tuning by injection current caused considerable spectrum line width broadening and a decrease in emitted power, both important criteria in DWDM applications. Further, the long term affects of wavelength tuning by injection currents on SSG-DBR laser performance remains unknown. In addition, current SSG-DBR lasers are monolithic devices fabricated from InGaAsP/InP and the manufacture of such SSG-DBR lasers results in low yield because of the immaturity of the InP or GaAs based processing technology. For a more detailed discussion of a wavelength tunable SSG-DBR laser by injection current, please refer to a paper by Ishii et al. entitled “Narrow Spectral Linewidth Under Wavelength Tuning in Thermally Tunable Super-Structure-Grating (SSG) DBR Lasers,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2, Pages 401-407, June 1995, which is hereby incorporated by reference.
For a more detailed discussion of the state of the art on widely tunable lasers, please refer to a paper by Rigole et al. entitled “State-of-the-art: Widely Tunable Lasers,” SPIE, Vol. 3001, Pages 382-393, 1997, which is hereby incorporated by reference.